It’s the brightest object in our night sky, but how far is the moon from Earth?
The moon‘s distance from Earth affects the strength of ocean tides and the appearance of solar eclipses in our skies.
The average distance between the blue planet and its only natural satellite is about 238,855 miles (384,400 kilometers), according to NASA.
The moon does not orbit Earth in a perfect circle, so there are places along its trip when it is either closer or farther away from our planet than this average distance.
The moon at perigee
When the moon reaches its shortest distance to Earth, or perigee, it is about 226,000 miles (363,300 km) away.
If perigee coincides with a full moon phase, this is generally called a supermoon. The term is not a scientific one, but it is used by the skywatching community when the full moon appears within 90% of perigee.
Supermoons appear roughly 17% bigger and 30% brighter than the faintest moon of the year. Since the moon’s gravity exerts a pulling force on Earth’s oceans to produce tides, the moon’s closer proximity to Earth around perigee can also trigger higher-than-usual tides.
The moon at apogee
When the moon is at its farthest distance from Earth, known as apogee, it is about 251,000 miles (405,500 km) from our planet. One of the clearest natural visuals for apogee are annular, or “ring of fire,” solar eclipses.
A remarkable quality about solar eclipses is the serendipity of their existence. For the next 600 million years, the moon’s proximity to Earth will be about 400 times greater than the sun’s distance to Earth. Since the star’s diameter is also approximately 400 times greater than that of the moon, the disks of both objects tend to match almost perfectly during solar eclipses. This produces moments of totality, when just the solar corona peeks out from behind the moon.
But if the new moon phase that produces solar eclipses coincides with apogee, the moon’s longer distance from Earth means that the lunar disk will appear much smaller in the sky. While still spectacular, annular solar eclipses do not completely block out the sun, Instead, they leave a ring of the solar disk visible to spectators.
How long does it take to get to the moon?
A range of factors determines how long it takes to reach the moon. Human missions, for instance, tend to take longer than passenger-free satellites. Whether or not an object stops at the moon or just zips by also comes into play.
The USSR launched the first mission to the moon, Luna 1, in 1959. With no propulsion system, the sphere-shaped satellite was hurled into space, and took only 34 flight-hours to make the trip. After its flyby, the satellite went into orbit around the sun, between the orbits of Earth and Mars. This remains one of the fastest trips to the moon.
In 2003, the European Space Agency launched SMART-1, the first successful European spacecraft to the moon. Rather than travel a direct path, SMART-1 spiraled around Earth to reach its satellite, arriving more than a year after launch. Instead of propellant, SMART-1 made the first use of an ion engine, in combination with gravity assist maneuvers, to reach the moon making it extremely fuel efficient. The extended path provided significant insight into the Earth-moon system
“Operating SMART-1 has been an extremely complex but rewarding task,” Octavio Camino-Ramos, ESA SMART-1 Spacecraft Operations Manager said in a statement. “The long spiraling trajectory around Earth to test solar electric propulsion (a low-thrust approach), the long exposure to radiation, the strong perturbations of the gravity fields of the Earth-moon system and then the reaching of a lunar orbit optimized for the scientific investigations, have allowed us to gain valuable expertise in navigation techniques for low-thrust propulsion.” He called the findings “a remarkable benchmark for the future.”
NASA sent eight crewed Apollo missions to the moon, six of which landed successfully. (Apollo 8 was the first mission to orbit another body and Apollo 13’s infamous disaster resulted in a journey around the moon rather than a landing on its surface.) Each spent about three days traveling through space.
Apollo 8 took 69 hours, 8 minutes to enter orbit around the moon. Apollo 11, which placed the first humans on the moon, took 75 hours and 56 minutes to enter orbit around the moon. Long before they entered orbit, however, both spacecraft entered the moon’s sphere of influence, a region 33,823 nautical miles (62,630 km) from the moon. For Apollo 11, this occurred after 61 hours and 56 minutes, while for Apollo 8 it took only 55 hours 40 minutes.
But the quickest trip to the moon was the New Horizons probe, which zipped past the moon in just 8 hours 35 minutes. The spacecraft didn’t even slow down or approach lunar orbit but instead zipped by on its way to Pluto.
The moon is slowly drifting away
The moon was once much closer to Earth. During the infancy of the solar system just as planets were finalizing their formation, a protoplanetary Earth collided with a Mars-sized object that may have formed farther away from the sun. Known as Theia, its impact produced the material that would eventually coalesce to form the moon.
The distance between the Earth and the moon grew as time passed. Currently the moon drifts away from our planet at a rate of approximately 1.5 inches (3.8 cm) a year, according to NASA.
Additional Resources
To learn more about the moon’s many facets, visit NASA’s Moon page.
Bibliography
Ahrens, Caitlin. “Origin of the Moon, Impactor Theory.” Encyclopedia of Lunar Science (2021).
Cano, Erick J., Zachary D. Sharp, and Charles K. Shearer. “Distinct oxygen isotope compositions of the Earth and Moon.” Nature Geoscience 13.4 (2020): 270-274.
Jones, Andrea, et al. “Overview | Inside & Out – Moon: NASA Science.” Public Engagement Team at NASA Goddard’s Solar System Exploration Division. Accessed 31 January 2022.
Jones, Andrea, et al. “Eclipses | Moon in Motion.” Public Engagement Team at NASA Goddard’s Solar System Exploration Division. Accessed 31 January 2022.
Jones, Andrea, et al. “Supermoons | Moon in Motion.” Public Engagement Team at NASA Goddard’s Solar System Exploration Division. Accessed 31 January 2022.
Jones, Andrea, et al. “Moon Phases | Moon in Motion.” Public Engagement Team at NASA Goddard’s Solar System Exploration Division. Accessed 31 January 2022.